In recent years, in order to record characters and/or a graphic written on a whiteboard or a blackboard as electronic data, a digital camera or a camera phone is sometimes utilized. Japanese Patent Application Publication No. 2006-121488 discloses a printer which processes color data obtained by taking a photograph with a digital camera. The printer reduces the number of gradation values of a color image inputted to the printer. This is because the inputted color image is originally provided with a large number of gradation values. As a result, a color image data of a smaller size is generated. Small-sized data is suitable for data transfer by e-mail or wireless communication.
Japanese Patent Application Publication No. 2000-36909 discloses an electronic blackboard which scans an image on a screen. The screen is included in the electronic blackboard. The read mage is then recorded on a recording sheet. In order to stabilize the quality of the image to be recorded, regardless of light source intensity and the density of the original image, the electronic blackboard can change a threshold value used for image reading.
Furthermore, Japanese Patent Application Publication No. 4-362850 discloses a color separator which color-separates the image obtained by reading an original image put on a white screen. The screen is included in the electronic blackboard. The color separator obtains monochrome image data by reading characters and/or a graphic written/formed in black and red on the screen. The color separator uses a monochrome image sensor represented by pixels having a brightness value in the range of 0 to 255 (256 values of gradation). The color separator determines the brightness value of each pixel to create a brightness histogram showing the relation between brightness value and the number of pixels at each brightness value. Based on the pixel distribution shown in the density histogram, the color separator achieves separation among the following colors of pixels: black, white, and red.
If the color image is obtained by photographing a whiteboard, the color image contains pixels having the ground color of the whiteboard. Since dust or dirt adheres to the surface of the whiteboard, the pixels which have the ground color do not represent perfect white. In order to print the color image, therefore, additional ink or toner is required for the ground color. On the other hand, if a color image is obtained by photographing a blackboard, since the ground of the blackboard has a color near black, a large amount of toner or ink is consumed for printing the color image. The problem is that, ink or toner is wastefully consumed for the ground color of an image, although the ground color is not significant information.
In addition, the color image printed on a recording sheet is disadvantageous. Such an image sometimes cannot obtain a printed result having a good visibility, due to dust or dirt adhering to the surface of the whiteboard or the blackboard.